wicket-util/src/main/java/org/apache/wicket/util/io/ThresholdingOutputStream.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.wicket.util.io;

import java.io.IOException;
import java.io.OutputStream;


/**
 * An output stream which triggers an event when a specified number of bytes of data have been
 * written to it. The event can be used, for example, to throw an exception if a maximum has been
 * reached, or to switch the underlying stream type when the threshold is exceeded.
 * <p>
 * This class overrides all <code>OutputStream</code> methods. However, these overrides ultimately
 * call the corresponding methods in the underlying output stream implementation.
 * <p>
 * NOTE: This implementation may trigger the event <em>before</em> the threshold is actually
 * reached, since it triggers when a pending write operation would cause the threshold to be
 * exceeded.
 * 
 * @author <a href="mailto:martinc@apache.org">Martin Cooper</a>
 * @version $Id$
 */
public abstract class ThresholdingOutputStream extends OutputStream
{

	// ----------------------------------------------------------- Data members


	/**
	 * The threshold at which the event will be triggered.
	 */
	private final int threshold;


	/**
	 * The number of bytes written to the output stream.
	 */
	private long written;


	/**
	 * Whether or not the configured threshold has been exceeded.
	 */
	private boolean thresholdExceeded;


	// ----------------------------------------------------------- Constructors


	/**
	 * Constructs an instance of this class which will trigger an event at the specified threshold.
	 * 
	 * @param threshold
	 *            The number of bytes at which to trigger an event.
	 */
	public ThresholdingOutputStream(final int threshold)
	{
		this.threshold = threshold;
	}


	// --------------------------------------------------- OutputStream methods


	/**
	 * Writes the specified byte to this output stream.
	 * 
	 * @param b
	 *            The byte to be written.
	 * @exception IOException
	 *                if an error occurs.
	 */
	@Override
	public void write(final int b) throws IOException
	{
		checkThreshold(1);
		getStream().write(b);
		written++;
	}


	/**
	 * Writes <code>b.length</code> bytes from the specified byte array to this output stream.
	 * 
	 * @param b
	 *            The array of bytes to be written.
	 * @exception IOException
	 *                if an error occurs.
	 */
	@Override
	public void write(final byte b[]) throws IOException
	{
		checkThreshold(b.length);
		getStream().write(b);
		written += b.length;
	}


	/**
	 * Writes <code>len</code> bytes from the specified byte array starting at offset
	 * <code>off</code> to this output stream.
	 * 
	 * @param b
	 *            The byte array from which the data will be written.
	 * @param off
	 *            The start offset in the byte array.
	 * @param len
	 *            The number of bytes to write.
	 * @exception IOException
	 *                if an error occurs.
	 */
	@Override
	public void write(final byte b[], final int off, final int len) throws IOException
	{
		checkThreshold(len);
		getStream().write(b, off, len);
		written += len;
	}


	/**
	 * Flushes this output stream and forces any buffered output bytes to be written out.
	 * 
	 * @exception IOException
	 *                if an error occurs.
	 */
	@Override
	public void flush() throws IOException
	{
		getStream().flush();
	}


	/**
	 * Closes this output stream and releases any system resources associated with this stream.
	 * 
	 * @exception IOException
	 *                if an error occurs.
	 */
	@Override
	public void close() throws IOException
	{
		try
		{
			flush();
		}
		catch (IOException ignored)
		{
			// ignore
		}
		getStream().close();
	}


	// --------------------------------------------------------- Public methods


	/**
	 * Returns the threshold, in bytes, at which an event will be triggered.
	 * 
	 * @return The threshold point, in bytes.
	 */
	public int getThreshold()
	{
		return threshold;
	}


	/**
	 * Returns the number of bytes that have been written to this output stream.
	 * 
	 * @return The number of bytes written.
	 */
	public long getByteCount()
	{
		return written;
	}


	/**
	 * Determines whether or not the configured threshold has been exceeded for this output stream.
	 * 
	 * @return <code>true</code> if the threshold has been reached; <code>false</code> otherwise.
	 */
	public boolean isThresholdExceeded()
	{
		return (written > threshold);
	}


	// ------------------------------------------------------ Protected methods


	/**
	 * Checks to see if writing the specified number of bytes would cause the configured threshold
	 * to be exceeded. If so, triggers an event to allow a concrete implementation to take action on
	 * this.
	 * 
	 * @param count
	 *            The number of bytes about to be written to the underlying output stream.
	 * @exception IOException
	 *                if an error occurs.
	 */
	protected void checkThreshold(final int count) throws IOException
	{
		if (!thresholdExceeded && (written + count > threshold))
		{
			thresholdReached();
			thresholdExceeded = true;
		}
	}


	// ------------------------------------------------------- Abstract methods


	/**
	 * Returns the underlying output stream, to which the corresponding <code>OutputStream</code>
	 * methods in this class will ultimately delegate.
	 * 
	 * @return The underlying output stream.
	 * @exception IOException
	 *                if an error occurs.
	 */
	protected abstract OutputStream getStream() throws IOException;


	/**
	 * Indicates that the configured threshold has been reached, and that a subclass should take
	 * whatever action necessary on this event. This may include changing the underlying output
	 * stream.
	 * 
	 * @exception IOException
	 *                if an error occurs.
	 */
	protected abstract void thresholdReached() throws IOException;
}